When printing it is desired that a printed color matches a color of source content as closely as possible, for example under any viewing or illumination conditions. Printing commonly uses a colorimetric approach to visually match the printed color to the color of source content. Colorimetry approximates human color perception by representing the color of observed surfaces or objects under a single set of viewing conditions using tristimulus values—one for each of the three types of light sensitive cells (cones) lining the retina at the back of the human eye. One type of cone cell is sensitive mostly to long wavelengths (L), another to medium wavelengths (M) and yet another to short wavelengths (S) of electromagnetic radiation in the visible range (i.e., from approximately 400 to 700 nm). Mostly for historical reasons, colorimetry uses a linear transformation of the LMS space, derived from psychophysical color matching experiments, called XYZ, defined by the CIE (Commission Internationale de l'Eclairage). The tristimulus values of X, Y and Z of a colorimetric approach form the basis of approximately representing all colors seen by a human visual system, tied to a set of viewing conditions, especially an illuminant present in the observed scene. Colors are thus identified based on co-ordinates in this CIE XYZ space.